The assessment of externalizing behavior (EB) in preschoolers is crucial in developmental psychopathology. In the absence of any gold standard measure, new tools contribute to a multi-method and multi-informant approach. The aim of the current study is to present and validate a new observational paradigm, the Unfair Card Game (UCG), intentionally structured to increase the likelihood that negative affect, agitation and inattention will emerge during a video-recorded task. It was administered to 268 young children, and the results were validated by means of factorial analysis, reliability analyses, inter-rater agreement, test–retest, discriminant analyses and external validation with the Child Behavior Checklist (Achenbach & Rescorla, 2004). The validity of the UCG is supported by the findings.